Application
VFA produced from carbohydrate-rich streams may serve as building blocks for the production of higher-value compounds compared to low-value biogas (Kleerebezem et al., 2015). Open culture fermentation can be used to produce organic acids from a complex substrate such as wastewater. Combining open culture fermentation with granular sludge technology, the production of a VFA rich effluent with a low solid content is possible. This is ideal for processes where the consumption of VFA results in a particulate endproduct like polyhydroxyalkanoates (PHA) that can be separated from the water phase (Jelmer Tamis, Lužkov, Jiang, Loosdrecht, & Kleerebezem, 2014). Tamis et al., (2014) demonstrated that minimizing the influent solid concentrations in the PHA production process enables maximization of the PHA content of biomass.
The granular sludge technology provides besides a low solid effluent, a high volumetric productivity. Uncoupling of the solid and liquid retention enables reduction of the bioreactor volume while maintaining similar output in terms fermentation products formed. This study showed the diversity of the product spectrum that can be obtained through control of the SRT. There was no mechanistic explanation found why in this study one product spectrum prevailed over others at different SRT.

Conclusion

This study showed the successive establishment of anaerobic granular sludge cultures enriched on glucose at 1-2 d, 10-20 d and 40-50 d SRT at a pH of 5.5. Two distinct product spectra were obtained (i) at 40-50 d SRT a propionate:acetate mixture of 2.05:1 (molpropionate:molacetate) was obtained; with a VFA production yield of 0.79 ± 0.12 (n=18) gCOD·gCOD-1. (ii) At 1-2 d and 10-20 d SRT an acetate dominated, 0.71-0.75 molacetate·molVFA-1, product spectrum was obtained. Overall, high VFA yields of 0.77-0.79 gCOD·gCOD-1 from glucose fermentations were obtained. Furthermore, compact sludge beds were obtained as SVI60 were ranging within 11-44 mL·gTSS-1, and Bifidobacterium scardovii was the prevailent microorganism in all systems. Substrate specific uptake rates varied from 0.2-0.7 gCOD·gVSS-1·h-1. Despite the relatively low qSmax, the glucose consumption rate of the systems varied from 100 gCOD·L-1·d-1 to a maximum rate of 1100 gCOD·L-1·d-1. Overall, this work showed the benefits of granular sludge technology for fermenting carbohydrate-rich water resulting in a VFA rich effluent with a low concentration solids. The possibilities of applying anaerobic granular sludge are just touched upon and more understanding is desired to control the product spectrum and granulation.

Acknowledgements

The financial support from the Dutch Applied Science foundation (NWO-TTW) and Paques BV through the VFA-platform program (Project No. 12998) is gratefully acknowledged.

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Tables
Table 1. Overview system characteristics obtained at different SRT. The YX/S was obtained by averaging the VSS values obtained in the effluent together with the biomass growth in the reactor and/or manual removal of biomass from the reactor.